1. Field of the Invention
This invention relates to a process for improving resistance of control rod guide tube split pins in nuclear reactors to stress corrosion cracking which comprises heating said split pin to a critical elevated temperature level, cooling at least the surface portions of the said split pin subject to stress corrosion cracking and then permitting said split pin to come to ambient temperature.
2. Description of the Prior Art
Split pins made of iron or alloys thereof are used in nuclear reactors to help position the control rod guide tube bottom flange in relation to the upper core plate and to provide lateral support and to maintain alignment of the guide tube with respect to the fuel assembly guide tube thimbles therein. Machining of these split pins during the manufacturing process results in split pins having high tensile residual stresses on the machined surfaces. When the split pins are assembled on the control rod guide tube bottom flange, and during the operation of the reactor, portions of the split pin develop further tensile stresses at various locations on said split pins. In a nuclear reactor these split pins are generally in a hostile environment, for example, in a pressurized water reactor wherein the water contains dissolved oxygen and chemicals that often remain even after the water has been demineralized. Under these circumstances those portions of the split pin that are under high tensile stresses are subject to stress corrosion cracking, particularly when such split pins are made of stainless steel or high nickel alloys. When this happens, the split pins so affected have to be removed and replaced with new split pins at great cost in time and money.